EP1370070A1 - Color management via reference gamut - Google Patents

Abstract

The method involves receiving image data representing first color space positions and describing first color values in an input color range, transforming to reference positions in a reference color space describing second color values, whereby a reference color range is determined that can be represented by the reference positions, and transforming to positions in a third color space describing color values in the output device's color range. The method involves receiving image data representing first color space positions and describing first color values in an input color range, transforming to reference positions in a reference color space describing second color values, whereby a reference color range is determined that can be represented by the reference positions, and transforming to third positions in a third color space describing color values within the output color range of the output device (Geraet 3,4). Independent claims are also included for the following: (a) a computer program for implementing the inventive method (b) a computer memory medium or computer with an inventive program (c) and a photo printer, color printer or photo laboratory.

Description

The present invention relates to the processing of image data in order to obtain as much as possible to achieve optimal color rendering. The invention particularly relates to the field of Photography, that is, the image data, in particular, represent photographic images as they are through image capture devices, such as photo cameras, video cameras, digital cameras, scanners etc. can be won. The image data are used to control image display systems, such as photo printers, photo laboratories, minilabs, color laser printers, inkjet printers, Monitors (liquid crystal monitors and CRT monitors), etc., are used. The present invention serves the color impression of the images by the image display system e.g. on a medium (paper, photo paper, foil etc.) or on a Screen (monitor) are shown to improve. The invention serves in particular by an input device (an image capture device, e.g. scanner, Digital camera etc.) detectable and / or outputable color space (input gamut) with to match the color space (output gamut) that can be represented by the image display system, if the input gamut does not match the output gamut. The The present invention thus relates to the processing of image data by an input device were recorded and are available in a device-dependent color space, so that them through an image display system that also has a device-dependent color space determines, can be output as optimally as possible.

An example of the scope of the present invention described above represents the processing of sRGB image data, for example from a digital camera are output by an image display system (e.g. photo printer or Minilab) can be processed so that the image created by the image data is shown, is produced with the help of a photo paper.

a) Die Übertragungsfunktion und die Chromatizität der primären Phosphorfarben von Kathodenstrahlröhren (CRT-Monitoren) ähneln dem sRGB-Farbraum sehr. Bilder können also in vernünftiger Qualität auf Monitoren gezeigt werden, ohne dass eine Abbildung der Farben mittels eines Profils zusätzlich erforderlich ist. Der sRGB-Farbraum ist inzwischen so allgemein üblich, dass sogar in Technikfeldern, deren Übertragungsfunktionen stark von sRGB abweichen, wie beispielsweise LCD-Monitore oder Plasmamonitore, diese immer noch die Bilddateneingabe über eine sRGB-Schnittstelle unterstützen.

b) Die Hersteller von digitalen Scannern, Monitoren und Digitalkameras stellen häufig als weiteres Merkmal ihrer Geräte bereit, dass diese sRGB-Bilddaten ausgeben.

c) Nahezu alle Computerprogramme, die herkömmlich erhältlich sind, unterstützen sRGB-ähnliche Farbräume.

The sRGB color space and its relationship, for example, with the XYZ color space is described, for example, in "The Creation of the sRGB ICC Profile" by Mary Nielsen and Michael Stokes, Hewlett-Packard Company, Boise, Idaho, USA, in Color Research No. 568, Pp. 253-257, 1998. The sRGB color space is particularly popular for the following reasons:

a) The transfer function and the chromaticity of the primary phosphor colors of cathode ray tubes (CRT monitors) are very similar to the sRGB color space. Images can therefore be shown on monitors in a reasonable quality without the need to additionally depict the colors using a profile. The sRGB color space has become so common that even in technical fields whose transfer functions differ greatly from sRGB, such as LCD monitors or plasma monitors, they still support image data input via an sRGB interface.

b) The manufacturers of digital scanners, monitors and digital cameras often provide as a further feature of their devices that they output sRGB image data.

c) Almost all computer programs that are commercially available support sRGB-like color spaces.

Unfortunately, the sRGB color space and color space of photo paper (e.g. Silver halide paper) significantly different (see Fig. 1). Wide areas of the sRGB color space, especially the bright, saturated colors are far outside the Gamut of photo paper. Conversely, a monitor usually fails because of the reproduce dark colors of photo paper. Also color spaces of inkjet and Color laser printers differ significantly from the sRGB color space.

a) Einige Komponenten des Photofinishingsystems sind hinsichtlich einer exakten Farbwiedergabe zu wenig stabil. Dies ist z.B. bei einem Fotopapierprozessor der Fall, dessen Stabilität den Anforderungen eines klassischen Farbmanagementsystems oft nicht genügt. Oder die Information über das Farbprofil des Eingangsgeräts, beispielsweise des Films, ist nicht mit ausreichender Genauigkeit bekannt oder kann sich häufig ändern.

b) Die Ausgangsgeräte verwenden nur die Farben, die die Ausgangs- und Eingangsgeräte gemeinsam haben. Somit wird nicht das volle Potenzial des Ausgangsgeräts verwendet. Die Erfinder haben jedoch erkannt, dass gerade im Fotobereich der Kunde den Farbeindruck eines Bildes als angenehmer oder positiver empfindet, wenn die Bildwiedergabe mit mehr Farben erfolgt, also das Farbpotenzial des Ausgangsgeräts besser nutzt. Dieser angenehme Eindruck ist für den Kunden wichtiger als eine kolorimetrisch exakte Wiedergabe der Bilddaten, wie dies in der Druckindustrie der Fall ist.

c) In jenen Bereichen des Farbraums des Eingangsgeräts, in dem die Farbwerte einer stärkeren Änderung unterzogen werden müssen, um in den Gaumut des Ausgangsgeräts zu passen, gehen die Details von Farbübergängen und Farbnuancierungen verloren.

d) Falls digitale Daten von einem Negativfilm oder vom Abtasten von Farbbildern gewonnen werden und in digitaler Form, beispielsweise als CD, ausgegeben werden, geht Farbinformation verloren, da der Gamut des Negativfilms oder der Ausdrucke bzw. Prints üblicherweise nicht mit dem Gamut des Scanners übereinstimmt. So ist üblicherweise das Ausgabeformat des Scanners sRGB. Falls in einem nächsten Schritt Reprints bzw. erneute Ausdrucke basierend auf den digitalen Daten vorgenommen werden, werden nicht dieselben Ergebnisse erhalten, wie wenn der Ausdruck direkt beispielsweise basierend auf dem Negativfilm vorgenommen worden wäre.

e) Die nummerische Realisierung von Farbverarbeitungsmodulen, die beim Farbmanagement eingesetzt werden und beispielsweise dreidimensionale Nachschlagtabellen (3D-LUTs) verwenden, haben häufig Probleme mit der Genauigkeit und der nummerischen Stabilität in der Nähe der Gamutgrenzen, da die dort vorgenommene Gamutabbildung singuläre erste Ableitungen bei der Gamutgrenze verursachen.

It is the field of color management to coordinate device-dependent color spaces. Typically, each device or device has its own color profile. The connection between input and output profiles is preferably made in color management by means of a color space that is independent of the input and output devices. This color space is also called a profile connection space (PCS). Input and output devices generally have a different gamut. The basic concept of color management is described, for example, in the article "Color Management: Current Practice and the Adoption of a New Standard" by Michael Has and Todd Newman, which is available at http://www.color.org/wpaperl.html can be. In the basic concept, called colorimetric match, the colors in both gamuts are converted as far as possible while maintaining the color value. Colors that cannot be generated by a specific source device are mapped to the gamut limit. The term "absolute rendering intent" is used for this method. Colorimetric adjustment compromises are usually made to adjust the white point of the two devices (relative rendering intent). Additionally further adjustments can be made, in particular by coordinating the darkest shades of gray of the two devices with one another (recognition / reproduction approach or "perceptual rendering intent"). The use of such a color management system and its standardization (ICC) takes place in the printing industry Approach to the photofinishing industry, however, has several disadvantages:

a) Some components of the photofinishing system are not stable enough with regard to exact color reproduction. This is the case, for example, with a photo paper processor, the stability of which often does not meet the requirements of a classic color management system. Or the information about the color profile of the input device, for example the film, is not known with sufficient accuracy or can change frequently.

b) The output devices only use the colors that the output and input devices have in common. This means that the full potential of the output device is not used. However, the inventors have recognized that, especially in the photo area, the customer perceives the color impression of an image as more pleasant or more positive if the image is reproduced with more colors, that is to say that it uses the color potential of the output device better. For the customer, this pleasant impression is more important than a colorimetrically exact reproduction of the image data, as is the case in the printing industry.

c) In those areas of the color space of the input device, in which the color values have to be subjected to a major change in order to fit into the palate of the output device, the details of color transitions and color nuances are lost.

d) If digital data is obtained from a negative film or from the scanning of color images and is output in digital form, for example on a CD, color information is lost because the gamut of the negative film or the prints or prints usually does not match the gamut of the scanner , This is usually the output format of the scanner sRGB. If reprints or reprints are made in a next step based on the digital data, the same results are not obtained as if the printout had been made directly, for example, based on the negative film.

e) The numerical implementation of color processing modules that are used in color management and, for example, use three-dimensional look-up tables (3D LUTs), often have problems with the accuracy and numerical stability near the gamut boundaries, since the gamut mapping performed there is a singular first derivative at the Cause gamut limit.

The object of the invention is to provide a color management system that is flexible to a wide variety of different types Input devices (image capture devices) and output devices (image display systems) can be customized.

The above object is achieved by the subject matter of claims 1, 9, 10 and 11. Further embodiments are the subject of the dependent claims.

The present invention relates to a method for processing image data, in particular a color management process. The image data represent color values of an image, in particular photographic image. By processing according to the invention the color values of the photographic image, in particular by an image capture device the image display options of an output device, especially an image display system (photo printer, photo laboratory, monitor, printer, Color laser printers, inkjet printers etc.) can be adjusted. The one that can be handled by one device Color space is called the color gamut of the device. The color gamut of Input device (e.g. image capture device) includes all color values from that Input device are detectable and can be output. The from the input device output image data which are processed by the method according to the invention, reflect the color gamut of the input device, especially the photographic one Information captured and / or processed. The process of the invention received image data can be received in a variety of ways, especially via data carriers, e.g. CD, networks, internet or through direct Connection to an image capture device, such as B. a scanner or a digital camera etc. The method according to the invention is intended to be the input into the method Image data are processed in such a way that the image data output by the method Display color values that are better adapted to the color gamut of the source device, than the image data entered in the method. The color gamut of the source device, So the so-called output color gamut preferably includes all color values can be processed by the output device when inputting image data into the output device (e.g. representable, storable and / or communicable).

As usual, the image data can be two-dimensional or three-dimensional (e.g. holograms) his.

The image data received by the method according to the invention represent so-called first ones Represent positions in a first color space and describe first color values. These color values have the property that they lie within the input color gamut.

According to the inventive method, the first are in a first step Positions are transformed by an input transformation into second positions, which as Reference positions are referred to. The input transformation is preferably done so that the reference positions are in a second color space. So it happens a transformation from a first color space into a second color space. Preferably the first color space and the second color space differ. The first color space is in particular a device-dependent color space of the input device and second color space is a device-independent color space, e.g. CIELAB or XYZ.

In a second step, the reference positions are changed by an initial transformation transformed into third positions. The transformation is preferably done that the third positions are in a third color space. So there is one Transformation from the reference color space into a third color space. Preferably differentiate the second color space and the third color space. The third color space is in particular a device-dependent color space, preferably to the output device is adjusted (for example an RGB color space). Preferably differs at least either the first or the third color space from the second color space. Preferably at least part of the first positions and / or first color values is different than the second positions and / or second color values. Preferably at least one Part of the third position and / or the third color values different from the second positions and / or second color values. Preferably at least either part of the first Position or the third positions other than the second positions. Preferably is at least either the input transformation or the output transformation no identity transformation or at most one of the transformations is an identity transformation. The output transformation can, for example, be an identity transformation if the output color gamut matches the reference color gamut. The input transformation can also be an identity transformation if the input color gamut matches the reference color gamut. For example the output device can be a data recording device or a network interface be that can record or transmit the image data that the reference color gamut include. Accordingly, the input device can be a data reader or a Network interface that can read or receive the image data that the Open the reference color gamut. In this way, data can reference positions represent, stored / sent and / or read / received.

However, the reference color gamut is preferably designed such that it does not match the Input color gamut is still identical to the output color gamut.

The input transformation according to the invention can at least be broken down into a transformation from the first color space into the second color space (called "first color space transformation") and into a mapping (called _" first mapping") within the second color space. The color value is preferably not changed during the transformation from the first to the second color space. The positions resulting from the transformation are referred to below as input transformation positions. The mapping then carried out within the second color space can at least partially lead to a change in color value. Although this breakdown of the input transformation into a transformation and a mapping is described below, this is purely exemplary, since, for example, the first color space transformation and the first mapping within the second color space can be mathematically combined into a single, related input transformation.

Likewise, the initial transformation according to the invention can at least be thought into a map (called "second map") within the second color space and into a transformation from the reference color space to the third color space (called the second Decompose the color space). The figure performed within the second Color space can at least partially lead to a change in color value. In contrast, is preferred in the transformation from the second to the third color space, the color value not changed. The positions shown in the figure are as follows referred to as initial transformation positions, which are then transformed into the third color space are transferred to the third positions. Although in the following this breakdown of the initial transformation into a mapping and transformation is described, this is purely exemplary, for example, mathematically the second mapping within the second color space and the second color space transformation combined into a single, related output transformation can be.

According to the inventive method, z. B. after the aforementioned Input transformation in the second color space an adaptation of the color values to one predetermined color gamut, which is referred to as the reference color gamut, by the first illustration. This color gamut serves as a "bridge" or "mediation" between the input color gamut and the output color gamut. The reference positions Both the color space and gamut are independent of the input type from the output device. So there is not only one color space platform, but also one "Gamutplattform". Preferably, those through the input transformation positions represented color values, which preferably correspond to the first color values, within of the reference color space to those described by the reference color gamut Color display options adjusted. This is preferably done using the first Figure that shows the input transformation positions in the reference color space to others Maps positions that are the reference positions mentioned above. The reference positions describe the reference color values in the reference color space. The illustration is designed so that the reference color values (also "second color values" called) lie within the reference color gamut (or on its border). The reference color gamut is preferably designed to take into account the color values of all drawn and / or predetermined input color gamuts and output color gamuts comprises at least approximately.

The reference color gamut preferably comprises in particular in addition to the input color gamut (or in addition to a union of a number or a plurality predetermined input color gamuts) at least parts of the output color gamuts (or a union of a number or a plurality of predetermined output color gamuts), those in the input color gamut (or in the union of a number or multitude of input color gamuts) are not included, and / or in addition to Initial color gamut (or in addition to the union of a number or Variety of output color gamuts) at least parts of the input color gamuts (or that of a union of a number or a plurality of predetermined input color gamuts), those in the original color gamut (or the union of a number or Variety of predetermined output color gamuts) are not included, and particularly includes preferably the starting color gamut (or the union of one Number or plurality of predetermined output color gamuts) and the input color gamut (or the union of a number or a plurality of predetermined input color gamuts) at least roughly. The reference color gamut preferably comprises Color values from the original color gamut (or the union of a number or Plurality of predetermined output color gamuts) included in the input color gamut (or the Union of a number or a plurality of predetermined input color gamuts) are not included, and / or color values from the input color gamut (or the union set a number or a plurality of predetermined input color gamuts) which are in the Output color gamut (or the union of a number or plurality of predetermined ones Output color gamuts) are not included.

The first image is preferably designed objectively within the reference color space become, that means in particular no color information is lost.

The third positions are then preferably used to control the image control data determine which are used to control the output device, the output color gamut having.

The reference positions then preferably serve as the basis for processed image data to win that serve to control an output device. The initial transformation is designed so that the third color values within the output color gamut (or on its border). Preferably the second mapping is inside of the reference color space can be designed bijectively, which means that none is possible Color information lost.

Both the input transformation and the output transformation are preferred designed bijectively. This is how the image data is processed reversible, so that no color information is lost even with a longer work chain goes. In particular, bijective transformations are advantageous when data generated by an input device, for example a film scanner on the one hand on an output device, For example, a photo printer, are output, and at the same time on a digital Medium, such as CD, are cached. If later pictures from this digital Medium again on an output device, in particular the same output device, output, it can be ensured that no color information is lost. To do this, the transformation when reading from a digital medium (is in this Function input device) of the inverse transformation of the writing (is in this Function output device) correspond to what presupposes bijectivity. According to the invention can use the same devices as both an input device and an output device become.

According to the invention, the input transformation and the output transformation, especially the first figure and the second figure within the Reference color space designed according to specified boundary conditions and / or properties. The reference color values are preferably at least substantially the same as first color values if the first positions (first color values) are in a predetermined Range or part of the first color space and / or if the input transformation and or reference positions (second color values) in a predetermined Area or subspace of the reference color space. In particular, the second Color values at least substantially equal to the first color values when the first and / or reference color values (also called "second color values") represent a medium gray or in the vicinity. There is preferably also no or no significant Change the color values when the color values (those by the first positions and / or reference positions are displayed) in the interior (interior subspace, which in particular comprises a medium gray) of the input color gamut and / or the reference color gamut. This interior is particularly of the interface of the reference color gamut and / or the input color gamut. The distance is, for example, less than 50, 30 or 10% of the minimum distance to the border input color gamuts and / or reference color gamuts.

The input transformation is therefore preferably position-dependent, that is to say z. B. dependent from the position of the first positions and / or the reference positions.

The first color values that lie on the interface of the input color gamut are by mapping the input transformation in the reference color space mapped onto the boundary (boundary surface) of the reference color gamut.

The reference color values are preferably at least substantially equal to the third Color values if the third positions (third color values) are in a given range or part of the third color space and / or if the output transformation and or reference positions (second color values) in a predetermined range or subspace of the reference color space. In particular, the reference color values at least substantially equal to the third color values if the reference color values and / or third color values represent a medium gray or are in the vicinity thereof. There is also preferably no or no significant change in the color values if the color values (represented by the third positions and / or reference positions be) in the interior (interior subspace, which is in particular a medium Gray comprises) of the starting color gamut and / or the reference color gamut. This interior is particularly from the interface of the original color gamut and / or the reference color gamut removed. The distance is less, for example than 50, 30 or 10% of the minimum distance to the input color gamut limit and / or reference color gamuts.

The output transformation is therefore preferably position-dependent, that is to say z. B. dependent from the position of the third positions and / or the reference positions.

The reference color values that lie on the interface of the reference color gamut are by mapping the initial transformation in the reference color space mapped to the boundary of the original color gamut.

A kind of plastic deformation of the input color gamut preferably takes place via the reference color gamut in the original color gamut. This deformation is preferably ie change in color value, as explained above, in the central area, in particular near the gray axis, not or only slightly formed, while the extent of the Color value change towards the edge of the input color gamut increases, especially in the dimensions of the input color gamut and the output color gamut in the corresponding Color value range are different. The first color values, which are inside the input color gamut and near the border of the Input color gamuts are mapped to a second color value that is within the The starting color gamut is and is approximately equally close to the border of the starting color gamut comes to rest.

As described above, there is a plastic deformation of the input color gamut preferred over the reference color gamut to the original color gamut. Different expressed, the neighborhood relationships are preferably maintained, where preferably the changes in the distances from neighboring color values by the illustration the closer the color values are to the boundary of the color gamut, the greater the color values and / or each different of the output gamut from the input gamut in this color value range is.

So far in this application of distances between color values or within one Color space is mentioned, this relates in particular to color differences as they appear CIE standards are defined that relate in particular to human color perception Respectively.

For example, the CIELAB color space is a color space that is adapted to the color sensitivity of the human eye. In the CIELAB color space, each color pair that is separated by a Euclidean distance 1 appears equally far apart from one another for a human viewer. Under ideal conditions, a trained viewer is able to distinguish colors up to approximately ΔE = [(ΔL ² + Δa ² + Δb ² )] ^½ = 1.

The first image within the reference color space is preferably designed such that an input transformation comprising the mapping is preferably continuous. Preferably the first derivative of the input transformation is also continuous and in particular not singular. In any case, this should be for the relevant value space (i.e. first Color values within the input color gamut and second color values within the reference gamut be valid.

The second image within the reference color space is preferably designed such that that an initial transformation comprising the second figure is preferably continuous is. Preferably, the first derivative of the output transformation is also continuous especially does not become singular. This preferably applies at least to the relevant one Value space (i.e. second color values within the reference gamut and third color values within the original color gamut).

The reference color gamut is preferably designed such that it does not only include the input color gamut an input device, but a plurality of input devices. As a result, image data originating from a large number of input devices can processed according to the inventive method for color management of image data become. The reference color gamut preferably comprises the starting color gamuts a variety of output devices. This enables flexible color management can be achieved for a variety of output devices. So preferably by the method according to the invention, the image data of a large number of input devices with different color gamuts flexibly to a desired source device can be adapted to a variety of output devices.

The first map and the second map are within the reference color space preferably designed so that the first, second and third color values are the same or at least have (very) similar hue. So it is preferably only a shift within a color plane in the reference color space on which the hue is constant. This maintains an essential aspect of the color information, while the dynamics of the output device, for example, in terms of brightness and / or color saturation can be fully used.

If the color values are manipulated, in particular local manipulations, that affect the color values in parts of the image, i.e. locally, so it is preferably done in the reference color space, preferably based on the reference positions and preferably taking into account the reference gamut. Based on the manipulated reference positions are then, for example, according to one of the The method described above determines the image control data. This has the advantage that the manipulation process independent of the input device and the output device can be used. Manipulation methods include in particular methods for local darkening and / or lightening of an image, procedures based on image content recognition are supported, such as methods for eliminating the red-eye effect, and methods based on the detection of "memory colors", such as skin tones. Procedures are particularly general includes that relates to local changes in image properties within the image, such as local change in sharpness, local change in color value, local change in brightness etc. are directed.

The present invention particularly relates to a program which, when on a Computer is carried out or carried out by a data processing unit is caused, the computer or the data processing unit, the method perform. In particular, the invention also relates to a computer storage medium, such as a CD, DVD, floppy disk, etc., which stores the above method or which contains information corresponding to the program. Further concerns the The present invention is a signal wave, the information as the aforementioned program contains and / or transported, in particular a signal wave that a transmission of Program over a network, such as the Internet.

The invention further relates to a computer on which the above-mentioned program is saved.

The invention also relates to a photo printer, for example with photo paper works (e.g. a DMD photo printer), color printers such as inkjet color printers or laser color printer, or a photo laboratory, especially a minilab, a laboratory with a particularly small footprint of just a few square meters or less than a square meter, or even a large laboratory. The aforementioned photo printer, Color printer or the aforementioned photo laboratory in particular comprises a unit to receive the image data. This unit is an interface to, for example Receiving data via a network, in particular the Internet, a memory reader, such as CD readers or memory card readers to the storage media read on which the image data (photographic images) are stored. Furthermore, a data processing unit, in particular a computer, is a motherboard with CPU or a CPU or an ASIC. This data processing unit processes the received image data according to the inventive method to the image control data for controlling an output device, in particular an image display system, especially an image recording system. The image recording system is in particular an exposure unit for exposing a photosensitive Photo paper according to the image control data, a color printer, in particular an inkjet printer or toner printer, for producing the photographic image on a Medium, especially paper or foil.

The invention also relates to input devices, for example film scanners, and output devices, For example, photo printers that are geographically separate and via a network (e.g. CAN or Internet) are connected. The image data is preferably transmitted as reference positions in the reference color space. The invention also relates to a System of input and output devices connected, for example, via a network are, the system using the inventive method. In particular The present invention relates to a device (input device and / or Output device) and / or method for storing and / or reading and / or Receiving, sending of data relating to the reference positions generated according to the invention represent. In particular, this method or the device for use in the aforementioned system, the input device being the left one Represents half in Fig. 2 and the output device represents the right half in Fig. 2 and the two devices using the reference positions (or data that the Represent reference positions) with each other (e.g. via a network or data carrier) communicate. The input device thus generates according to the invention Process generates the reference positions from the image data and the output device the image control data according to the invention from the reference positions Method. The input device preferably comprises at least one input device, a data processing device to the received image data according to the invention Process to convert reference data to the reference positions represent, and a data storage device to the reference data on a disk save, and / or a reference data transmission interface. The output device preferably includes a reference data reader to retrieve the reference data from the data carrier to read, and / or a reference data reception interface, a data processing device, order from the received reference data according to the invention Method to determine the image control data and an output device by the Image control data can be controlled.

Fig. 1

zeigt die Kopplung verschiedenster Eingangsgeräte mit verschiedenen Ausgangsgeräten über eine Farbraumplattform gemäß dem erfindungsgemäßen Verfahren;

Fig. 2

zeigt das erfindungsgemäße Farbmanagementverfahren;

Fig. 3

zeigt einen sRGB-Farb-Gamut und einen Fotopapier-Gamut im zweiten Farbraum;

Fig. 4

zeigt den Verlauf einer Wichtungsfunktion, die bei der Gewinnung der Referenzpositionen eingesetzt wird;

Fig. 5

zeigt eine Abbildung im zweiten Farbraum, bei der der Farbton erhalten bleibt;

Fig. 6

zeigt eine weitere Abbildungsfunktion, um die Referenzpositionen zu erhalten.

Further advantages and properties according to the invention are disclosed in the following detailed description. Different features of different designs can be combined with each other.

Fig. 1

shows the coupling of various input devices with different output devices via a color space platform according to the inventive method;

Fig. 2

shows the color management method according to the invention;

Fig. 3

shows an sRGB color gamut and a photo paper gamut in the second color space;

Fig. 4

shows the course of a weighting function, which is used in the acquisition of the reference positions;

Fig. 5

shows an image in the second color space, in which the color tone is retained;

Fig. 6

shows another mapping function to obtain the reference positions.

1 shows a networking of input devices and output devices via a color space platform. A desired color space or standard space is used as the color space platform used, which is referred to above as the reference color space. Preferably used as the reference color space a CIE color space, such as a CIELAB color space or a CIEXYZ color space chosen, as is common in today's color management systems These have the desired properties that they are independent of the type of input device or output device and that it is the color spaces of all possible input devices and output devices. If you put the received image data in Reference color space, for example, by placing them in the reference color space transformed, if they do not already exist in the reference color space, you put found that the color values (first color values) represented by the image data are not complete are independent of the type of device, as these are related to the detectable colors restricted (e.g. camera or film scanner). In other words, the first color values reveal the properties of the input gamut of the input devices. The reference color space is preferably designed so that it has its own color value and assigns a well-defined position or a single point in the color space. Preferably there are defined transformations between the first and the Reference color space and in particular between the reference color space and the third Color space. Source devices are, in particular, imaging systems, such as dye systems (Photo printer or color printer), monitors, storage media or network interfaces.

2 shows the color management according to the invention from the perspective of the color spaces. The Input devices are referred to as device 1 and device 2. The device 1 is for example a digital camera that outputs color data in the form of the sRGB format. The Device 2 is, for example, a scanner that scans a negative film and stores the data as Outputs RGB data. The image data output by device 1 are referred to as RGB1 and the image data output from the device 2 is referred to as RGB2. The RGB1 and RGB2 each have their own color space. By the invention They are transformed into the reference color space. The one shown in Fig. 2 Example is the CIELAB color space. Within this reference color space the transformed image data now mirror the input gamut of the device 1 or device 2. In other words, all possible input transformation positions span the input color gamut of the input device in the second color space on. A particular idea of the method according to the invention now lies in this Restriction imposed on the input transformation positions by an illustration (first figure) to overcome the space of possible color values in the Maps the reference gamut. In this way, properties of the input device (Color properties) removed or at least weakened.

The output transformation positions of the output devices 3 and 4 define analogously the output color gamut of device 3 and 4 in the reference color space. The image (second figure) of the initial transformation again forms a rule like that second positions on the output color gamut of devices 3 and 4. Then the color values are mapped in the device-dependent color space RGB3 and RGB4. In this way, properties of the output device (color properties) so taken into account that the color rendering within the possibilities becomes optimal. The reference color space and the first associated with each input device Illustration, as well as the second illustration belonging to each output device leads to an intermediary between the input device and the output device. As a result of that multiple input devices and thus multiple input color gamuts and multiple output devices and thus linked several output color gamuts via the color space platform high flexibility with a wide range of applications achieved.

A bijective, that is to say invertible for each color value, is preferably used for each input device Transformation from the input device color value (first positions) to the Reference color value (second positions) defined.

A bijective, that is to say invertible for each color value, is preferably used for each output device Transformation from the reference color value (second positions) to the output device color value (third positions) defined.

The input transformation from the first color space into the reference color space is preferred colorimetrically exact if the color values are far from the gamut limit are. The input transformation takes place, for example, from RGB to CIELAB. The Colors on the gamut border (e.g. the RGB cube area) are indicated by the Input transformation preferably from the surface of the RGB color cube to that of the reference color gamut. Colors will be near the gamut border preferably deformed similar to a plastic deformation, with neighboring ones Color values always remain adjacent color values. The first derivative of the transformation should not become singular to a bijective property of transformation achieve. For numerical reasons, the first derivative should be within a given one Remain value range.

The reference positions are preferably directly by means of an RGB-CIELab transformation calculated. The image control data representing the third positions are preferably calculated by first computing an RGB-CIELab transform which is then inverted.

The advantages of the invention are, in particular, that no or at least only a little Gamut mapping needs to be done, resulting in a loss of color detail and can lead to numerical problems at the gamut boundaries. In addition, the Transformation can be inverted for each color value. After all, there is no color information lost.

A series of color value changes, for example images within the second color space are possible. For example, the size of the interior the reference color gamut for which no change in color value is to take place, can be varied. With regard to the change in color value, in particular with regard to the Different methods are possible for color values that lie on the edge of the gamut. For example the weight can be used to maintain or change the color components, Color saturation components or brightness components can be varied.

The reference color gamut can be chosen in a variety of ways. For example, the gamut from XYZ, that from sRGB, the gamut of a typical output device or even selected a geometrically well-defined gamut (such as a sphere) become. However, the volume of the reference color gamut should preferably be approximate equal to or greater than the volume of the input and output color gamuts his.

When processing the image data, the chain of transformations (first color space transformation, first mapping, second mapping, second color space transformation) step by step, one transformation after another. The chain the transformation can be run in the order mentioned, as also in the opposite direction (reverse order), as desired and Aim. However, preferably two, three or four neighboring transformations can also be carried out be combined into one transformation around the image of fewer transformations to submit, which enables faster processing. In particular can the input transformation (first color space transformation and first mapping) with the output transformation (second image and second color space transformation) linked to a single transformation.

3 shows the gamut of the sRGB as well as the input color gamut in the second color space as the starting color gamut, the gamut of photo paper. As you can see, they overlap the two gamuts in broad areas. But there are also parts of the sRGB gamut outside of the photo paper gamut and vice versa. According to the invention, the Reference color gamut for both the sRGB gamut and the photo paper gamut.

In the following, a combination of the gamut of photo paper with the sRGB gamut as the reference color gamut is used as the first example. For this purpose, a transformation from a paper RGB color space into the CIELAB color space is preferably defined. This can be done, for example, by a simple grid or a mesh, which has the dimensions 65 x 65 x 65, for example. Intermediate points are defined by linear interpolation. Alternatively, a mathematically defined function can be used. For each color triple of RGB there is a corresponding CIELAB triple called Lab _pRGB . The calculation of the CIELAB triple from the RGB triples is described below.

In the example described, sRGB is used as an example to match the colors of the Represent input device. Thus, the standard formula can be used to create a Transformation from sRGB to CIELAB. In addition to the sRGB color space other RGB color spaces can also be used. However, are preferred Transformations from this color space into the second color space (e.g.-CIELAB) known.

The transformation can be defined, for example, as follows, where Lab stands for a triple value: rennet weighted = (1 - w RGB ) · Lab sRGB + w RGB · Lab pRGB

The weight W _RGB is a function of the closest distance d _RGB from an sRGB date (first position) to the sRGB surface. w _RGB = 1 for colors on the surface of the RGB cube and w _RGB = 0 in the middle of the RGB cube. The distance d is normalized to 1 if it denotes the distance from the surface of the cube to the center of the cube.

A typical weighting function is as follows:

The w _i used in the above formula is, for example, w _RGB and the above d _i is, for example, the d _RGB and "i" denotes the i th sRGB date. The course of w _i is shown in FIG. 4 as a function of d _i . As you can see, the "plastic deformation" is almost or not present near the center and then increases from around 30% of the maximum distance to the outside. So for large distances, the influence of the output reference gamut increases with increasing distance, whereas the influence of the input reference gamut decreases with increasing distance, but at least predominates for small distances.

w_i = 1,0 auf der Oberfläche;

w_i = 0,0 in der Mitte des Würfels;

w_i steigt monoton an;

die erste Ableitung von w_i nach d_i ist für alle 0 ≤ d_i ≤ 1 nicht singulär und insbesondere innerhalb eines vorgegebenen Wertebereichs, der sich für eine (vorgegebene) numerische Verarbeitung eignet.

Other weighting functions that meet the following conditions may also be suitable:

w _i = 1.0 on the surface;

w _i = 0.0 in the middle of the cube;

w _i increases monotonously;

the first derivative from w _i to d _i is not singular for all 0 d d _i 1 1 and in particular within a predefined range of values which is suitable for (predefined) numerical processing.

An example is described below in which the reference gamut is the full one XYZ gamut includes.

The gamut of the XYZ includes all possible colors. The transformation is in three Steps described:

The transformation takes place from CIELAB to XYZ according to a standard formula. If the input data are not yet in the CIELAB form, a transformation from CIELAB to XYZ is carried out, as described in the previous example. The XYZ values resulting from this transformation L _sample values are referred to as xyz _sample .

Next, the xyz _sample values are transformed into xyz _mapped values while preserving the hue. Within the XYZ half-planes, which contain the gray axis (x, x, x) with x = 0 ... 1, all colors are described that have a certain hue. Figure 5 shows such a half-plane that contains this gray axis. All color values in the half-plane shown in FIG. 5 that do not lie on the gray axis have the same hue.
The third step is a transformation back into the CIELAB color space according to a standard formula. The xyz _mapped values are thereby transformed into L _mapped values.

In summary, the transformation xyz _sample to xyz- _mapped in half-planes with a constant hue is described. For each color value with the position xyz _sample you can define a straight line that goes through two points. One point is called the reference point xyz _reference (for example 0.5, 0.5, 0.5) and the other is called xyz _sample . This line crosses the input color gamut (RGB gamut) at the point xyz _RGB-Surface and the XYZ cube at xyz _cube .

The mapping function applies to all points on the straight line: xyz _mapped = f (xyz _sample ). This function can be defined with the following boundary conditions: xyz _RGB-surface is mapped on xyz _cube and points near xyz _reference can remain unchanged. In addition, as mentioned above, the hue should remain unchanged, which is ensured a priori by making all color shifts within the half-plane.

6 shows a typical mapping function for f. The solid line denotes the course of the function f as a function of xyz _sample . The dashed line denotes the case of a 1: 1 mapping, _i.e. xyz _sample = xyz _mapped , which is the case when the input RGB gamut is equal to the reference gamut in the corresponding direction.

Claims (10)

Method for color management of image data that represent the color values of an image in order to achieve the best possible change in the color values with regard to different and given color gamuts of an input device, in particular a film scanner or digital camera and an output device, in particular a photo printer or photo laboratory, one input Color gamut comprises all color values that can be recorded by the input device and output as image data, and an output color gamut includes all color values that can be processed by the output device when image control data is input into the output device, with the following steps: a) the image data representing first positions in a first color space and describing first color values which lie within the input color gamut are received; b) the first positions are transformed by an input transformation into reference positions, the transformation being designed such that the reference positions lie in a reference color space and describe second color values, a reference color gamut specifying the color values that can be represented by the reference positions; c) the reference positions are transformed into third positions by an output transformation, the output transformation being designed in such a way that the third positions lie in a third color space and describe third color values which are encompassed by the output color gamut of the output device. The method of claim 1, wherein the input transformation is designed such that a) the second color values are at least substantially equal to the first color values if the color values are around a medium gray or in the vicinity thereof and / or if the first and / or second color values in the interior of the input color gamut and / or reference Color gamuts are distant from the interface of the input color gamuts and / or reference color gamuts; and or b) the first color values lying on the interface of the input color gamut are mapped to second color values lying on the interface of the reference color gamut; and or c) the neighborhood relationships between the color values are maintained, with first color values that are adjacent being mapped into second color values that are also neighboring; and or d) the first derivative of the input transformation does not become singular and / or the input transformation is bijective. The method of claim 1 or 2, wherein the output transformation is designed such that a) the third color values are at least substantially equal to the second color values if the color values are around a medium gray or in the vicinity thereof and / or if the second color values and / or third color values in the interior of the starting color gamut and / or reference Color gamuts are distant from the interface of the starting color gamut and / or reference color gamut; and or b) the second color values lying on the interface of the reference color gamut are mapped third color values lying on the interface of the starting color gamut; and or c) the neighborhood relationships between the color values are maintained, with second color values that are adjacent being mapped into third color values that are also neighboring; and or e) the first derivative of the output transformation does not become singular and / or the output transformation is bijective. The method of claim 1 or 2 or 3, wherein the reference color gamut a variety of input color gamuts a variety of input devices and / or a variety of output color gamuts from a variety of output devices includes. Method according to one of claims 1 to 4, wherein the input and / or Output transformations is designed so that at least part of the second Color values have the same hue as at least part of the first and / or third color values. Method according to one of the preceding claims, in which the image data, that represent the reference positions in the second color space are optimized before they are further processed to obtain the image control data. Method according to one of the preceding claims, in which the input transformation and the output transformation combined into one transformation are. Program that, when loaded into or running on a computer, causes the computer to carry out the method according to one of claims 1 to 7. Computer storage medium or computer with a program according to claim 8. Photo printer, color printer or photo laboratory, especially a large laboratory or minilab,
with a unit for receiving the image data;
with a data processing unit which processes the received image data according to the method according to one of Claims 1 to 7 in order to obtain the image control data;
with an image recording system that generates a photographic image based on the image control data on a recording medium, in particular paper or photo paper.

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